1. Field of the Invention
The present invention generally relates to communication terminal equipment and, more particularly, relates to communication terminal equipment which is driven by power supplied through a communication line after communication between a switching system and the communication terminal equipment is established through the communication line.
2. Description of the Background Art
Generally, a telephone set has so far been adapted to be driven by power supplied only from a telephone line connected thereto.
FIG. 3 is a schematic diagram showing a structure of a conventional telephone set to which a data terminal equipment can be connected.
In the figure, a telephone set 6 is connected to a switching system 1 through a telephone line 4 and further connected to a data terminal equipment 3 through a terminal connecting cable 5. Switching system 1 supplies power to each telephone set connected thereto through line 4, and also is required for driving the telephone set as well as controlling communication connection between a telephone set and another telephone set through line 4. Telephone set 6 is driven by power supplied only from switching system 1 through telephone line 4. Telephone set 6 includes a power supply circuit 21, an audio system circuit 22 for transmitting/receiving an audio signal and a data system circuit 24 for transmitting/receiving data. Data terminal equipment 3 such as a microcomputer having a function of transmitting/receiving data is connected to data system circuit 24 through terminal connecting cable 5. Terminal equipment 3 transmits/receives data to/from data system circuit 24. Power supply circuit 21 supplies power provided from connecting line 4 connected thereto to audio system circuit 22 and data system circuit 24 connected to the subsequent stage.
In the operation of telephone set 6 shown in FIG. 3, when line connection between telephone set 6 and switching system 1 is established through telephone line 4 and communication with another telephone set is made possible, then switching system 1 starts providing power to telephone set 6. The power is supplied to audio system circuit 22 and data system circuit 24 from power supply circuit 21. Audio system circuit 22 receives the power supplied from power supply circuit 21, transmits/receives an audio signal through telephone line 4 to carry out its talking process. Data system circuit 24 receives the power supplied from power supply circuit 21, controls transmission/reception of data between telephone line 4 and data terminal equipment 3, and further carries out data conversion and so on so that the data can be processed in data terminal equipment 3.
As described above, telephone set 6 shown in FIG. 3 receives the power supplied only from telephone line 4 and can carry out the talking processing and a data communication processing for data terminal equipment 3 at the same time. Since telephone set 6 can supply driving power to data system circuit 24 and audio system circuit 22 only with the power supplied from telephone line 4, it is not necessary to connect telephone set 6 to another power supply through an AC power adapter and so on, so that, advantageously, the requirement of an interconnection for that part can be saved and a decrease of cost can also be realized.
As described above, the power required for driving telephone set 6 is supplied only from switching system 1 through telephone line 4. As switching system 1 can supply a predetermined amount of power, a maximum level of power which can be supplied to each telephone set connected to switching system 1 is determined for itself. Accordingly, if a problem such as a short-circuit occurs inside circuit 24 of telephone set 6 or in a connector (not shown) connecting circuit 24 to terminal equipment 3, the amount of the power supplied from switching system 1 via telephone line 4 exceeds the above-mentioned maximum power level, doing damage to the function of circuit 24 or terminal equipment 3. In order to avoid such a damage before it happens, switching system 1 is adapted to stop supplying power to telephone set 6 on detecting the amount of power supply exceeding the above-mentioned maximum power level. That is, in the conventional telephone set 6, if a problem such as a short-circuit occurs inside data system circuit 24 or in, a connector connecting circuit 24 to terminal equipment 3 and so on, the power required for driving telephone set 6 is immediately cut off. There was a problem that when the driving power itself was cut off, not only the function of transmitting/receiving data on the side of data system circuit 24 but also talking by voice on the side of audio system circuit 22, where no problem had occurred, could not be carried out, so that the availability of telephone set 6 was significantly decreased.
This is a problem to be considered particularly for telephone set 6 having data system circuit 24 as well as circuit 22 compared with a normal telephone set having audio system circuit 22 only because the cause of the trouble in data system circuit 24 built in telephone set 6 is rather more significant in data terminal equipment 3 connected to telephone set 6 and connecting cable 5 used for connection with data terminal equipment 3 than in telephone set 6 itself.